Vulcanizable rubber mixes

ABSTRACT

The present invention relates to vulcanizable rubber mixes which, apart from the rubbers, contain O,O-bis-(alkyl)-dithiophosphoric acid polysulfides and primary and/or secondary amines, which are characterized in that they may be vulcanized without release of nitrosamines with a high crosslink density and a high proportion of short sulfur bridges.

FIELD OF THE INVENTION

[0001] The present invention relates to vulcanizable rubber mixes whichmay be vulcanized without the release of nitrosamines with a highcrosslink density and a high proportion of short sulfur bridges.

BACKGROUND OF THE INVENTION

[0002] It is known from the prior art to prepare rubber vulcanizateswith a high degree of crosslinking by vulcanizing the correspondingrubbers in the presence of sulfur in combination with knownvulcanization accelerators such as sulfenamides, thiurams or carbamates.A disadvantage of using such vulcanization systems, however, is thatrelatively large amounts of crosslinking chemicals are required toobtain a high crosslink density in the vulcanizates. Anotherdisadvantage is that the proportion of long-chain sulfur bridges in thevulcanizate is relatively high, which means that the heat resistance ofthe vulcanizates is unsatisfactory. We refer in this connection, e.g.,to M. R. Kreijsa, J. L. Koenig, “The Nature of Sulfur Vulcanization” inElastomer Technology Handbook, ed, by N. P. Cheremisinoff, p. 475 ff,CRC Press Inc., Boca Raton 1993; A. D. Thorn, R. A. Robinson, “CompoundDesign” in Rubber Products Manufacturing Technology, ed., by A. K.Bhowmick, M. M. Hall, H. A. Benarey, p 1 ff, Marcel Dekker Inc., NewYork 1994, and A. K. Bhowmick, D. Mangaraj, “Vulcanization and CuringTechniques” in Rubber Products Manufacturing Technology, ed., by A. K.Bhowmick, M. M. Hall, H. A. Benarey, p. 315 ff, Marcel Dekker Inc., NewYork 1994.

SUMMARY OF THE INVENTION

[0003] An object of the present invention is to provide vulcanizablerubber mixes which produce vulcanizates which have a high crosslinkdensity and a high proportion of short-chain sulfur bridges. Moreover,the aim of the present invention is to execute vulcanization withvulcanization chemicals which do not release nitrosamines and aretherefore ecologically acceptable. Moreover, the aim of the presentinvention was to obtain a high crosslink density of the vulcanizateswith the lowest possible molar additions of crosslinking chemicals.

[0004] The present invention provides, therefore, vulcanizable rubbermixes containing

[0005] a) rubbers,

[0006] b) O,O-bis-(alkyl)dithiophosphoric acid polysulfidescorresponding to the formula

[0007] wherein

[0008] x represents 2, 3, 4 or 5 and

[0009] R represents a C₈-C₁₂-alkyl or -cycloalkyl radical

[0010] and

[0011] c) primary and/or secondary amines corresponding to the formula

[0012] wherein

[0013] Y represents hydrogen or a mercaptobenzothiazole radical,

[0014] R₁ represents hydrogen, C₁-C₆-alkyl, C₅ or C₆-cycloalkyl orC₇-C₁₂-aralkyl and

[0015] R₂ has the same meaning as R₁,

[0016] with the proviso that R₁ and R₂ do not simultaneously representhydrogen,

[0017] wherein the components b) and c) are present in a molar ratio offrom (0.5 to 1.5):1 and in a total amount of from 1.0 to 10 parts by wt.per 100 parts by wt. of rubbers in the rubber mixes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 shows a graph of Rheometer experiments of the vulcanizationaccelerators individually and in combination at 150° C.

[0019]FIG. 2 shows a graph of Rheometer experiments on the SdiOPindividually and in combination with equimolar amounts of CHA and DCHAat 150° C.

DETAILED DESCRIPTION OF THE INVENTION

[0020] A molar ratio of the components b) and c) from (0.9 to 1.1):1 ispreferred, and a total amount of from 1.0 to 7 parts by wt. per 100parts by wt. of rubbers in the rubber mixes.

[0021] The rubbers a) which may be used according to the presentinvention may contain double bonds corresponding to iodine values of atleast 2, preferably 5 to 470. The iodine values are generally determinedby the Wijs method (DIN 53241, part 1) after addition of iodine chlorideto acetic acid. The iodine value defines the amount of iodine in g whichis chemically bound by 100 g of substance.

[0022] The rubbers generally have Mooney viscosities ML 1-4/100° C. (DIN53523) of 10-150, preferably 20-120.

[0023] Rubbers used may be both natural rubber and synthetic rubbers.Preferred synthetic rubbers are described, for example, in I. Franta(Elastomers and Rubber Compounding Materials, Elsevier, New York 1989)or in Ullmanns Encyclopaedia of Industrial Chemistry volume A 23, VCHVerlag, Weinheim 1993. They include, i.a. BR = polybutadiene ABR =butadiene/acrylic acid C₁—C₄-alkyl ester copolymers, IR = polyisoprene,NR = natural rubber, SBR = styrene-butadiene copolymers with styrenecontents of 1-60, preferably 2-50 wt. %, XSBR = Styrene-butadienecopolymers and graft polymers with other unsaturated polar monomers suchas acrylic acid, methacrylic acid, acrylamide, methacrylamide, N-methoxymethyl methacrylic acid amide, N-acetoxy- methyl methacrylic acidamide, acrylonitrile, hydroxyethylacrylate and/orhydroxyethylmethacrylate with styrene contents of 2-50 wt. % andcontaining 1-20 wt. % of polar monomers polymerized into the molecule,IIR = Isobutylene/isoprene copolymers with isoprene contents of 0.5-10wt. %, BRIIR- = Isobutylene/isoprene copolymers with bromine contentsbrom- of 0.1-10 wt. %, inated CL/IIR = Chlorinated isobutylene/isoprenecopolymers with bromine contents of 0.1-10 wt. %, NBR =Polybutadiene-acrylonitrile copolymers with acrylonitrile contents of5-60, preferably 10-50 wt. %, HNBR = Partially hydrogenated NBR rubberin which up to 98.5% of the double bonds are hydrogenated, XHNBR = Fullyhydrogenated NBR rubber in which up to 100% of the double bonds arehydrogenated, EPDM = Ethylene-propylene-diene copolymers, FKM =Fluororubber, CR = Chloroprene rubbers, CM = Chlorinated polyethylene,CSM = Chlorosulfonated polyethylene, ACSM = Chlorosulfonatedpolyethylene/polypropylene, ECO = Epichlorohydrin rubber, ACM = Acrylicrubber, EAM = Vamac ® = copolymers of ethylene, methyl acrylate and athird carboxyl group-containing component (DuPont), ECO =Epichlorohydrin rubber, Q = Silicone rubbers, AU = Polyester urethanepolymers, EU = Polyether urethane polymers, and mixtures of saidrubbers.

[0024] Substances used as component b) in the rubber compounds accordingto the present invention are, in particular, those in which x representsthe numbers 3, 4 or 5 and R represents an alkyl or cycloalkyl radicalwith 8 carbon atoms. More preferably, component b) used isO,O-bis(2-ethylhexyl)dithiophosphoric acid polysulfide corresponding tothe formula below:

[0025] Primary and secondary amines used are preferably thosecorresponding to the above mentioned formula (component c) in which Yrepresents hydrogen or a mercaptobenzothiazolyl radical and R₁ and R₂have the meaning of C₅- or C₆-cycloalkyl. More preferably, primaryand/or secondary amines used are cyclohexylamine, dicyclohexylamine, CBS(N-cyclohexyl-2-benzothiazole sulfenamide), TBBS(N-tert.-butyl-2-benzothiazole sulfenamide) and DCBS(N,N,-dicyclohexyl-2-benzothiazole sulfenamide).

[0026] Of course, it is possible to use the components a), b) and c)both individually and as a mixture thereof in the rubber compoundsaccording to the present invention. In order to determine the mostfavorable mixing ratio, appropriate preliminary tests may be carriedout, the mixing ratio depending in particular on the later intended useof the rubber vulcanizates.

[0027] The vulcanizable rubber compounds according to the presentinvention are usually prepared by mixing the corresponding rubbers withthe components b) and c) described above in the given amounts insuitable mixing apparatus such as internal mixers, rolls or extruders.

[0028] In order to prepare corresponding vulcanizates, the rubbercompounds according to the invention are vulcanized in the usual way inthe presence of crosslinking agents (vulcanizing agents) such as sulfur.The amount of crosslinking agents such as sulfur which is added to therubber compounds to be vulcanized is about 0.1 to 6, preferably 0.5 to3.0 wt. %, based on the rubber.

[0029] Of course, further rubber chemicals of the kind mentioned orrubber auxiliaries may be added to the rubber mixes according to thepresent invention, as described, for example, in Khairi Nagdi:“Gummiwerkstoffe”, A manual for users, Vogel-Verlag, Würzburg 1981.These additives are added in the known amounts and depend on theintended use of the rubber vulcanizates in each case.

[0030] The present invention also provides the use of the rubber mixesaccording to the invention for the manufacture of rubber molded parts ofall kinds, particularly for the manufacture of tires, hoses, dampingcomponents, seals and profiles.

[0031] It is surprising that, with the rubber mixes according to thepresent invention described above, it is possible to prepare rubbervulcanizates which have a high crosslink density combined with a highproportion of short-chain sulfur bridges, and to keep the molar amountof vulcanizing agents (crosslinking agents) low compared with the priorart.

EXAMPLES

[0032] The NR-based test mixes examined (Crepe 1) are unfilled andcontain no plasticizers or antioxidants: TABLE 1 Component of mix Amount(phr) NR (Crepe 1) 100 ZnO  2.5 Stearic acid  1.0 Sulfur  1.7Accelerator Molar amount of sulfur in accelerator chemicals------------------------------------------------------- = 0.18 Molaramount of elemental sulfur

[0033] For the O,O-bis(alkyl)dithiophosphoric acid polysulfide, theamount of free sulfur was reduced to 1.4 phr in order to adjust thetotal sulfur concentration to 1.7 phr according to the formulation,compared with the other accelerators. For reasons of clarity, the nameO,O-bis(alkyl)-dithiophosphoric acid polysulfide is replaced in theExamples by SDiOP. For the same reasons, cyclohexylamine is abbreviatedto CHA and dicyclohexylamine to DCHA.

Example 1

[0034] Rheometer experiments on the vulcanization acceleratorsindividually and in combination at 150° C. (See FIG. 1).

[0035] Rheometer Behavior TABLE 2 Accelerator S″_(max)-S″_(min) [dNm]SdiOP 6.4 DCBS 6.9 SDiOP + DCBS 9.0

[0036] Kinetics TABLE 3 Accelerator t_(i) [min] K_(i) [min⁻¹] E_(a)[kJ/mole] SDiOP 7.0 0.22 107 DCBS 9.4 0.25  93 SDiOP + DCBS 8.3 0.68  91

[0037] The rheometer behavior (rheometer curves, rheometer data andkinetics) is determined in accordance with DIN 53529. The apparatus usedis an RPA 2000 from Apha Technologies. The kinetic data k_(i), t_(i) areobtained from a first-order formal kinetic evaluation in the intervalt₂₅ to t₆₅. Ea is obtained from regression by way of the k values at130, 140, 150, 160 and 170° C.

[0038] It follows from the tables that the combination according to theinvention has a markedly increased torque difference(S′_(max)−S′_(min)), that the activation energy E_(a) was reducedsimultaneously and the reaction rate constants were reduced whilst theinduction time lies between the two individual components.

Example 2

[0039] Determination of the crosslinking efficiency and crosslinkdensity of the vulcanization accelerators individually and incombination at 150° C.

[0040] Crosslink Structure, Crosslink Density TABLE 4 S₁ S₂ S_(x)Efficiency Density Accelerator [%] [%] [%] δve/δc (S) ve.10⁵/mole cm⁻³SDiOP 64 24 12 0.21 11.5 DCBS  0 40 60 0.28 12.6 SDiOP + DCBS 72 15 130.32 17.5

[0041] The crosslink density is determined according to P. J. Flory, J.Rehner, jr., J. Chem. Soc. 521 (1943). The crosslink structure isdetermined according to B. Saville, A. A. Watson, Rubber Chem. Technol.100 (1967). S₁=monosulfide linkage, S₂= disulfide linkage,S_(x)polysulfide linkage.

[0042] It follows from the values that the crosslinking efficiencyδve/δc is surprisingly high and at the same time the proportion ofshort/monosulfide sulfur bridges is very high. In addition, thecrosslink density that can be achieved in the chemical combinationaccording to the invention is disproportionately high.

Example 3

[0043] Rheometer experiments on the SDiOP individually and incombination with equimolar amounts of CHA and DCHA at 150° C. (See FIG.2).

[0044] Rheometer Behavior TABLE 5 Accelerator S″_(max)-S″_(min) [dNm]SdiOP 6.4 SDiOP + CHA 7.0 SDiOP + DCHA 7.8

[0045] Kinetics TABLE 6 Accelerator t_(i) [min] K_(i) [min⁻¹] E_(a)[kJ/mole] SdiOP 7.0 0.22 107 SDiOP + CHA 2.0 0.86  98 SDiOP + DCHA 2.20.57 115

[0046] It follows from the values in the tables that a surprisingactivation of the crosslinking reaction is obtained by the use of theamines according to the invention. This is evident from the greatertorque difference S∝_(max)−S′_(min), the shorter induction times t_(i)and the very greatly increased reaction rate constants k_(i).

Example 4

[0047] Determination of the crosslink density of SDiOP individually andin combination with equimolar amounts of CHA and DCHA at 150° C.

[0048] Crosslink Structure, Crosslink Density TABLE 7 S₁ S₂ S_(x)Density Accelerator [%] [%] [%] ve.10⁵/mole cm⁻³ SDiOP 68 21 11 12  SDiOP + CHA 48 21 31 13   SDiOP + DCHA 73 16 11 16.3

[0049] It follows from the values in the tables that the total crosslinkdensity as a whole is increased and that, in particular, the proportionof monosulfide crosslinks is very greatly increased when SDiOP is usedin combination with DCHA.

[0050] Although the invention has been described in detail in theforegoing for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be limited by the claims.

What is claimed is:
 1. Vulcanizable rubber mixes comprising: a) rubbers,b) O,O-bis-(alkyl)dithiophosphoric acid polysulfides corresponding tothe formula

wherein x represents 2, 3, 4 or 5 and R represents a C₈-C₁₂-alkyl or-cycloalkyl radical and c) primary and/or secondary amines correspondingto the formula

wherein Y represents hydrogen or a mercaptobenzothiazole radical, R₁represents hydrogen, C₁-C₆-alkyl, C₅ or C₆-cycloalkyl C₇-C₁₂-aralkyl andR₂ has the same meaning of R₁, with the proviso that R₁ and R₂ do notsimultaneously represent hydrogen, wherein the components b) and c) arein a molar ratio from (0.5 to 1.5):1 and are present in a total amountof from 1 to 10 parts by wt. per 100 parts by wt. of rubbers in therubber mixes.
 2. Rubber molded products comprising vulcanizable rubbermixes comprising: a) rubbers, b) O,O-bis-(alkyl)dithiophosphoric acidpolysulfides corresponding to the formula

wherein x represents 2, 3, 4 or 5 and R represents a C₈-C₁₂-alkyl or-cycloalkyl radical and c) primary and/or secondary amines correspondingto the formula

wherein Y represents hydrogen or a mercaptobenzothiazole radical, R₁represents hydrogen, C₁—C₆-alkyl, C₅ or C₆-cycloalkyl C₇—C₁₂-aralkyl andR₂ has the same meaning of R₁, with the proviso that R₁ and R₂ do notsimultaneously represent hydrogen,

wherein the components b) and c) are in a molar ratio from (0.5 to1.5):1 and are present in a total amount of from 1 to 10 parts by wt.per 100 parts by wt. of rubbers in the rubber mixes.
 3. A rubber moldedproduct according to claim 2 , wherein said rubber molded product isselected from the group consisting of tires, hoses, damping components,seals and profiles.